Wire bonding is defined herein as the process of interconnecting points in an electronic circuit using bare, small diameter wires or ribbons which are usually made of gold or aluminum. The installed wire lengths are attached at either end by temporary contact with a tool which provides a combination of force and/or heat and/or ultrasonic energy to effect a weld of the wire to the bond surface or termination means. Wire bonds are generally divided into two main categories of wedge-wedge and ball-wedge. A wedge bond or termination results from forces applied perpendicular to a long axis of a wire to "pinch" it against the surface being bonded or the bond surface. In contrast, a ball bond is formed by first having a length of wire protruding from a small hole in the tip of a conical tool or "capillary". This wire length, known as the "tail", is then melted into a ball that is, of course, larger than the original wire diameter. In order to attach this mass to a bonding surface, the tool tip engages the periphery of the ball and applies the bond forces along the same axis as the length of the wire.
A ball-wedge bonder uses a sequence of forming a ball, attaching the ball, trailing wire from the ball through the hole in the capillary to form the wire's length and shape in the circuit. The other end is attached by using the rounded face surrounding the hole or capillary of the very same tool to then pinch the wire, thereby forming a wedge bond.
In ball-wedge bonding, two techniques are prevalent for melting the length of wire extending from the capillary tip. In each case the process is referred to as "flame-off". One approach or technique is to use a hydrogen torch and the other is called electronic flame-off. The electronic flame-off technique uses an electrode which approaches the wire and discharges a high voltage arc to melt the wire into a ball. The duration and intensity of the arc are usually controllable parameters set to values which accomplish sufficient melting of the wire to form an appropriate size ball.
Additional information relative the above background may be found in U.S. Pat. Nos. 3,006,067 and 3,087,239.
A nominal set of values in a flame-off of a one mil diameter gold wire requires about two milliseconds at a current of four milliamps to form a ball 2.5 times the wire diameter. In such a situation, the high voltage arcing source may initially provide voltages up to 2,000 volts, but the arc, once established, maintains a voltage drop of about 400 to 500 volts. This drop in voltage is monitored by most electronic flame-off systems to flag improper flame-offs due to the wire being absent from the capillary (an open circuit condition) or being too long (and thus causing a short circuit). The error signal often causes an audible warning in a manual bonder or halts the operation of an automatic bonder.
As the melted ball draws against the capillary during flame-off, the heat is dissipated rapidly into the capillary tip and the ball melts no further. Any further application of the high voltage arc accomplishes nothing but metal sputtering and carbonizing from the ambient air. The disadvantage to allowing further arcing is the adverse effects related to a coating of the wire material on the capillary tool.
In the prior art, there have been two approaches to controlling ball size. In the tail length limited mode, the amount of wire protruding from the capillary is carefully controlled by some mechanical means, and then an excess of arc energy is provided to guarantee that all the protruding wire is consumed. Although this produces reduced tool life due to thermal shock, there is accurate ball size control and the ball nests and conforms to the tool tip and is well centered to allow accurate targeting of the bond.
The other approach is an energy limited mode, wherein the electronic flame-off duration and current is controlled to act on an excess length of wire. In this case, the melt does not contact the capillary tip of the tool and no thermal shock is introduced, so tool life is prolonged. However, the ball may occasionally fail to center under the tool properly during bonding, and thus increases the likelihood of an improper bond in the next bonding operation.
It will be apparent to those skilled in the art that the length of the tail extending from the capillary at the time of commencement of the arc will affect the ball size. Accordingly, the ball size needs to be held consistent so that the force, time or ultrasonic intensity applied to the subsequent ball bond will provide proper adhesion and prevent overlap of the ball onto adjacent conductive areas.
The referenced co-pending application involves detecting the occurrence of contact of the ball with the capillary tip and terminating the arc voltage. The present invention, on the other hand, measures in the alternative, the time between application of the arc and the contact of the ball or the duration of the arc, as applied by a timer, to provide a monitor of tail length. If tail length varies substantially, quality of the ball bonding process is likely to be inconsistent. Further, the tail length is an important factor in setting up the referenced values of force, time and ultrasonic intensity. Thus, the monitoring capability of the present invention is useful, not only in setting up the initial variables to commence the ball bonding operation, but also to make sure that alterations in the process occurring during the manufacturing stage do not deviate enough to substantially affect quality of the finished product.
It is accordingly an object of the present invention to provide apparatus for monitoring the tail length of the wire used to form the ball in a ball bonder.